1. Field of the Invention
Systems consistent with the present invention relate to transmitting and receiving a communication signal having an intra-guard-interval (IGI), and more particularly, to transmitting and receiving a communication signal having an IGI inserted to prevent intra-symbol interference and inter-symbol interference and to prevent performance degradation.
2. Description of the Related Art
Spread spectrum communication is a technology to transmit a signal by spreading the signal's spectrum to a far wider spectrum. Spread spectrum communication includes a method using a narrowband carrier with a certain period as in a code division multiple access (CDMA) scheme, and a method using a wideband carrier. The method using the narrowband carrier modulates the frequency to make the frequency band of the information to be transmitted smaller than the frequency band of the carrier signal. The method using the wideband carrier modulates the frequency to make the frequency band of the information to be transmitted larger than the frequency band of the carrier signal.
The carrier used in spread spectrum communication is typically a sine wave or a pulse. Recently, for the IEEE 802.15.4a standard, a method of transmitting information using a chaotic signal has been suggested.
The IEEE 802.15.4a task group is the location-aware low-power sensor network standardization group. The location-aware low-power sensor networking is a next generation communication technique in which the location awareness capability and the low power are added to the combination of IEEE 802.15.4 ZigBee and IEEE 802.15.3 ultra wide band (UWB) communication.
It is a chaotic signal modulation method that is suggested for the realization of the low power. The chaotic signal modulation can be designed in a simple radio frequency (RF) structure by hardware, and does not require circuits for a voltage controlled oscillator (VCO), phase locked loop (PLL), mixer and the like that have been requisite for existing RF products. Thus, the chaotic signal modulation can reduce the power consumption to 5 mW which is about one third of the preceding power consumption.
Representative modulation schemes of the chaotic signal modulation are a differential chaos shift keying (DCSK) scheme, and a pulse position based-chaotic modulation (PPB-CM) scheme.
The DCSK scheme shows the best bit error rate (BER) characteristics in the chaotic signal modulation. The DCSK scheme utilizes a reference signal. In the symbol duration, two chaotic sample parts corresponding to one data bit are transmitted. The first sample part is used as the reference signal and the second sample part is used as a data signal to transmit. The second sample part is generated by transmitting the reference signal, or by inverting and transmitting the reference signal according to whether a transmitted binary symbol is 0 or 1. In doing so, when the binary symbol is 0, the second sample part, that is, the data signal is generated by reversing the reference signal. When the binary symbol is 1, the data signal is generated by transmitting the reference signal. A receiving side extracts digital data by correlating the received two sample parts.
FIGS. 1A and 1B are graphs showing a state of the transmission data signal after passing through a channel according to the DCSK scheme of the related art.
As shown, after passing through the channel, the reference signal and the data signal are received through the multipath. The reference signal smears to the data signal area and interferes with the data signal, and the multipath signal generated in the data signal smears to the reference signal area and interferes with the reference signal of the next symbol duration. As a result, in the related art, a guard interval is inserted between symbols to prevent inter-symbol interference, that is, the interference of the data signal with the reference signal of the next symbol duration. The guard interval is an empty symbol duration which carries no data between the symbols.
The guard interval can reduce inter-symbol interference, but not intra-symbol interference, that is, the interference between the reference signal and the data signal. In practice, when measuring the reference signal and the data signal after passing through the channel at 2.5 Mbps, the channel 37 suffers from intra-symbol interference of 2.73% and the channel 38 suffers from intra-symbol interference of 88.2%.
However, according to the characteristics of the DCSK scheme, since the data is determined depending on the correlation between the reference signal and the data signal, the relationship between the reference signal and the data signal is significant. Thus, the interference between the reference signal and the data signal causes serious performance degradation.
By contrast, the PPB-CM scheme splits the symbol duration into two parts and transmits the chaotic signal in one of the two parts according to whether the binary symbol is 0 or 1. When the binary symbol is 1, the first half of the symbol duration carries the chaotic signal, and when the binary symbol is 0, the second half of the symbol duration carries the chaotic signal. Accordingly, the receiver extracts the digital data based on where the chaotic signal is loaded within the symbol duration.
FIG. 2 is a graph showing a state after the chaotic signal, transmitted according to the PPB-CM scheme of the related art, passes through the channel.
As shown in FIG. 2, the chaotic signal corresponding to the binary symbol 0 is loaded in the second half of the symbol duration. When the chaotic signal is received in the multipath, it affects the chaotic signal of the subsequent symbol period. That is, the inter-symbol interference arises. In this case, it is hard to accurately extract the digital data in the subsequent symbol period. The inter-symbol interference can be attenuated by inserting the guard interval between the symbol periods. However, disadvantageously, the insertion of the guard interval extends the transmission time as much as the guard interval.
In contrast, since the chaotic signal of the binary symbol 1 is loaded in the first half of the symbol period, when the chaotic signal is received in the multipath, the signal smears to the second half of the symbol period. That is, the intra-symbol interference arises. Hence, it is hard to accurately extract the digital data in the symbol period.
Such intra-symbol interference appears not only in the DCSK scheme and the PPB-CM scheme using the chaotic signal, but also in most of the modulation schemes using a conventional carrier. Therefore, it is desirable to prevent the intra-symbol interference to thus avoid performance degradation.